I had a similar problem and the font item in the reader menu wasn't changing the font size in the reader.  I found I had to go to Tools>Options>Incoming Mail>Message Reader and uncheck, Where a choice of format exists, display the fancy version.

When I set Pegasus to "Include the text of the original message in the reply," the text included almost always shows up in 10 pt. When I begin to compose the reply, that text shows up in 12 pt and in the font in which I normally write. Also, if the text of more than one previous message is included in the reply, any messages previous to the immediately preceding message (the one to which I am actually replying) show up in 12 pt. I cannot find a setting that controls this. I would appreciate any advice that anyone can offer. Many thanks for your help.

 Jules 

Reader font and size and Editor font and size are set separately. When you start a reply with quoted msg text your reader font is used and when you begin Edit or start new message your Editor font and size are used. Use the big f on the top main toolbar to set each preference while working with that function.

Replies to HTML messages are using your default if none specified in the HTML source but when specified that font is used until you begin to create new text.

Arial 10pt was the original default Reader and Editor font setting until changed. Arial does not render as large as most other fonts in the same size. Try setting something like Verdana 12pt in Reader and something like Consolas 12pt in Editor if you prefer larger monospaced font. 

Jerry,

Many thanks for explaining this. I will follow these directions while looking forward to a better outcome.

 Jules